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THE COMPLEMENT SYSTEM The complement system is a set of plasma proteins that act in a cascade to attack and kill extracellular pathogens. Approximately 30 components: –activating molecules –regulator factors –complement receptors –membrane proteins which inhibit the lysis of host cells Most of the complement proteins and glycoproteins are produced in the liver in an inactive form (zymogen). Activation is induced by proteolytic cleavage.